This invention relates to kaolin clay pigments and to the production thereof. In particular, the invention relates to a novel scheme for processing kaolin clay that results in unique ultrafine (nanosized) kaolin clay particles that have a smooth surface and have a substantially round shape. The invention relates also to the use of the new clay pigments in producing quality ink jet printing paper.
It is known that surface characteristics of paper (or any other printing surface) play a large role in how ink will be received and appear after application to the printing surface. Thus, varying print appearances can be expected depending on whether the surface ink is being applied to is uncoated or coated. Printing on uncoated paper results in low quality printing while printing on coated paper results in a higher quality print albeit of varying quality according to the nature of the paper coating compositions.
Kaolin clay is widely used, and has been used for many years, as a pigment to coat various paper products. In general, finer size fractions of kaolin clay are used to coat paper when a high gloss surface finish is desired. Coarser fractions, including delaminated grades, are used when opacification is sought. Typically, #2 coating clays are about 80% by weight finer than 2 microns; #1 coating clays are typically about 90% by weight finer than 2 microns; fine high glossing (FHG) clays are about 50% finer than 0.5 microns. These sizes are conventionally measured by sedimentation techniques; these measurements assume a spherical shape although fine particle size kaolin particles, especially those finer than 2 microns are platy and not spherical.
Kaolin products tend to have a wide distribution of sizes within a designated range. This is referred to as a xe2x80x9cpolydisperse particle size distribution.xe2x80x9d For example, in the case of commercial FHG kaolin pigments, the finest grades may contain many particles in the range of 0.3 to 2 microns although median size is about 0.3 microns.
It is well known that particle size distribution has a significant effect on the properties of the kaolin pigment as well as the utility in printing by conventional means such as rotogravure. More specifically narrow particles size distribution is known to affect particle packing. Narrow particle size distribution influences the porous structure of the coating, resulting in enhanced performance and often improves the printability. However, narrow particle size is frequently associated with undesirably high viscosity. To the best of our knowledge, narrowing of particle size distribution has been utilized with conventional coating clay fractions, delaminated and nondelaminated, especially these intended for use in gravure printing.
The particle shapes of kaolin pigments vary depending among other things on the clay source and processing, especially processing that is reflected in particle size distribution. Thus, individual clay platelets are typically flat particles finer than 2 microns (determined by sedimentation). Aggregates of kaolin clay platelets tend to have a higher ratio of diameter to thickness. Mechanical or chemical delamination of kaolin (originally composed of stacks of individual platelets) results, as expected, in an increase of the diameter thickness ratio. The ratio of diameter to thickness is conventionally referred to as xe2x80x9caspect ratioxe2x80x9d. In some cases, aspect ratio is calculated from sedigraphic data.
With the advance of transmissions electron microcopy (TEM), aspect ratios of kaolin can be measured with better accuracy and for fine particles such as the one described in this invention aspect ratios were determined by the method as stated in the reference xe2x80x9cKaolin Aspect Ratios Determined by Automated Microscopy and Electron Energy Loss Spectroscopyxe2x80x9d by Vanderwood et al. In Process Mineralogy XIII, The Minerals, Metals and Materials Soc. 1995. The method involves image acquisition and analysis techniques to determine the platelet diameters, combined with parallel electron energy loss spectroscopy to measure thickness of the same platelets. In this method, kaolin samples were dispersed, deposited on a TEM (Transmission Electron Microscope) grid and allowed to dry. The TEM grid loaded with kaolin particles was then examined under TEM and several hundreds of particles were imaged and their average diameter determined under computer control. The samples and their images were then transferred to a transmission electron microscope equipped for Parallel Electron Energy Loss Spectroscopy (PEELS). Previously sized particles were then subjected to PEELS analysis, and the degree of electron scattering for each particle was recorded. A calibration curve of the electron scattering vs. particle thickness was used to determine the particle thickness from PEELS data. All the data were then subjected to off-line statistical analysis for the summarization and presentation of results.
One of the newest uses of specialty coated paper is in modern ink jet paper. This is a uniquely demanding application where quality paper is sought, especially when multicolor printing is used. Matte or low gloss coated paper for ink jet printing are produced by applying porous pigment particles and a binder to the paper. A recent advance in the art of ink jet pigments is described in commonly assigned U.S. Pat. No. 5,997,625, Londo et al and is commercially available under the trademark DIGITEX. The teachings of the ""625 patent are incorporated herein by cross-reference.
In accordance with a preferred embodiment of the invention of the ""625 patent, the coating pigment composition comprises:
(a) a hydrous clay; preferably hydrous kaolin clay
(b) a caustic leached calcined kaolin clay; and
(c) a porous material having a pore size of up to 100 Angstroms and a BET surface area in the range of 200 to 1000 m2/g; preferably zeolite Y,
wherein (a), (b) and (c) are present in relative amounts such that said pigment composition when present in a coating produces substantially equivalent ink contact angles when said coating receives more than one type of colored ink.
Advantages of the compositions of the invention of the ""625 patent over the prior art porous silica include improved rheology and higher coating solids. This allows the high speed paper and coating machines to produce an ink-jet coated grade of paper not previously capable of being produced. The material described in the ""625 patent also offers a substantial reduction in cost over the silica pigment. Yet another unexpected result is that this pigment requires less coatweight and binder than the conventional silica coatings. Also, acceptable coatings can be made without the need for dispersants.
Two of the more important characteristics to be controlled in color ink jet printing are depth of penetration and feathering or bleeding of the ink when applied to the paper. Too deep of a penetration results in poor color intensity. Bleeding results in poor printing definition. A further criterion is to control the contact angle of the various ink jet colors (i.e., cyan, magenta, yellow and black) in a manner that the inks will substantially have the same contact angle when applied to the coated paper. When the contact angles of the various inks are substantially the same, the appearance of the ink colors are more uniform, i.e., one color does not appear more dull or more bright than another color.
Prior to this invention, an expensive non-pigmented overcoat of an hydrophilic resin was applied to the inkjet paper to achieve gloss. The overcoat was selected to avoid interaction between the overcoat and the inks so that the ink is accessible to the receptor coated paper. One reason for using a non-pigmented overcoat is that conventional paper coatings formulated with traditional kaolin pigments and organic binders and applied to paper at conventional coating weights, can adversely affect ink-jet printability.
The increasing demand for high print quality from ink jet printers presents challenges to the coating pigment industry not encountered with the coated paper used in printing and writing paper market segments. The silicate pigment system described in the patent to Londo et al, OPTICILxe2x80x943186, meets the challenge to provide a receptor coating that is especially beneficial when multicolor printing is used. However, the full exploitation of the benefits of DIGITEX pigment and other ink receptive pigments used in ink jet printing, such as silica, awaits the development of improved TOP coatings to achieve gloss without the expense of known resin-based top coats.
One embodiment of this invention comprises a novel ultrafine kaolin pigment capable of achieving superior glossy characteristics on coated paper including ink jet paper. The new kaolin coating pigment is extremely fine. Average size is below 0.2 microns as determined by sedimentation. Thus, most if not all of the particles are nanosize. The particles achieve circularity (roundness) with an average aspect ratio of 2.8 or below, generally in range of 1 to 2, preferably as close to 1 as possible, indicating the ratio of diameter to thickness is low. Preferably, at least 90% by weight of the particles have an aspect ratio below 4 and from 10% to 50% by weight have an aspect ratio of 1 to 2. Surface area (BET method using nitrogen as adsorbate) is greater than 25 m2/g. This is higher than that of known glossing grades of kaolin. Preferably, surface area is in the range of 27-30 m2/g, most preferably 28-29 m2/g. The novel kaolin particles, when viewed by scanning electron microscope have the appearance shown in the attached figure. The photomicrograph in the figure show that the particles are generally round and free of attached fine particles.
The kaolin pigment of this invention is especially useful as a transparent over-coated pigmented (TOP) layer that achieve the gloss comparable to currently available glossy ink jet photographic paper which contains an expensive non-pigmented over-coat of hydrophilic resin. There is no interaction between the jet inks and the TOP layer. In other words, the TOP layer does not form a barrier between the ink and the ink jet pigment coating.
A desirable feature of the new pigment is that it can be formed into a high solids (e.g., 70% solids) dispersed aqueous slurry. The high solids coating makes possible higher productivity in a paper mill because of less demand on the dryer and improved coat weight control. Conventional calendering conditions can be used for full gloss development. Another significant advantage is that the pigmented TOP coating can be applied at low coat weight (for example 1 to 4 gms/sq.m.) Use of low coat weight provides a more permeable coating layer which facilitates required penetration of the ink to the inkjet coating.
Also in accordance with this invention, a fine particle size, high purity kaolin clay fraction, preferably a bleached FHG coating clay, is dispersed in water, charged to a Netzsch disc mill with a particulate medium, preferably round silica, and processed in the mill until at least 90% by weight is finer than 0.5 micron. Multiple passes in the mill are preferably carried out to achieve the desired reduction in particle size, narrowing of particle size distribution and reduction in aspect ratio.
The Netzsch Disc Milling System, manufactured by Netzsch Inc, 119 Pickering Way, Exton, Pa. 19341 Model: LMC 60(E) SS, was used in practicing this invention. The function of the disc is to accelerate the grinding media centrifugally from the outer edge and centripetally from the inner edge of the disc. This creates shearing forces between the disc and the media. This agitation is carried to the surrounding media radially and axially by the kinetic energy imparted to the media by the disc.
A two zone spiraled jacket coats the system. Various separators are available to separate the grinding media (beads) from the product.